Not Applicable
Not Applicable
Not Applicable
The proposed concept is related to conversion of liquid and gaseous hydrocarbon and alcohol fuels to product gas containing hydrogen, carbon monoxide, and traces of hydrocarbons that is useable in fuel cells. In particular, it relates to the unique capability of internal combustion engines (ICEs) operated with fuel in excess of the stoichiometric quantity to carry out this fuel conversion process.
The background of the invention includes processes and systems for supplying fuel to fuel cells, the use of internal combustion engines as chemical reactors, and power plants combining these elements.
Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surface of electrodes. Conventionally, the oxidant gas is oxygen or air, and the fuel gas is hydrogen or a mixture of hydrogen, carbon monoxide, and traces of hydrocarbons. The fuel gas may also contain non-fuel gases including nitrogen, water vapor and carbon dioxide. The specific fuel gas composition requirements depend on the type of fuel cell. Low temperature fuel cells, exemplified by proton exchange membrane (PEM) cells and alkaline fuel cells (AFC), can only utilize hydrogen as fuel, and contain precious metal catalysts that are poisoned by carbon monoxide. High temperature fuel cells, exemplified by solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC), do not contain precious metal catalysts, and utilize hydrogen, carbon monoxide, and traces of hydrocarbons as fuel. Most fuel cell types are adversely affected by sulfur compounds.
Pure hydrogen is the ideal fuel for all fuel cell types, but it is not widely available. Further, storage and transportation involves large, heavy and costly means such as compressed gas bottles. Practical fuel cell generators must therefore utilize commonly available and easily transported fuels including natural gas, liquefied petroleum gas (LPG), methanol, ethanol, gasoline and diesel fuel, and logistic fuel. These hydrocarbons and alcohols must be reformed to fuel gas suitable for the particular fuel cell application. In addition, these fuels often contain sulfur that must be removed. Conventional processes for desulfurizing and reforming liquid and gaseous fuels are well known in the art, and will only be summarized.
Fuel reforming is based on the endothermic reaction of hydrocarbon or alcohol fuel with steam and/or CO2, to form CO and H2. This can be done in two ways. The first is steam reforming. Steam reformers use high temperature catalyst filled tubes heated by burners fueled by fuel cell exhaust fuel and air streams. Steam is supplied by a waste heat boiler. Heat transferred across the tube wall drives the endothermic reaction. Such systems provide the highest hydrogen yield, but tend to be large, complex, and slow to start up and respond to load changes. Further, they require sulfur removal from the feedstock to avoid catalyst poisoning. The second is partial oxidation (POX) reforming. POX reformers and catalytic autothermal reformers eliminate high temperature heat exchangers by reacting a rich mixture of fuel and air to provide the reforming heat within the gas stream. Steam is added to the hot hydrogen and carbon monoxide to cool the stream and increase hydrogen yield. Non-catalytic POX reformers operate at temperatures around 1000xc2x0 C. for gasoline and up to 1400xc2x0 C. for heavy hydrocarbons, necessitating special heat-resistant materials. Autothermal reformers use a catalyst to operate at temperatures under 1000xc2x0 C., and may be less costly. These systems are smaller, simpler and faster responding than steam reformers, and are preferred for applications such as vehicle propulsion. Even so, there is a delay before power is available in a cold start and the feedstock must be low in sulfur.
Generally heavier liquid hydrocarbons such as diesel fuel are the most difficult to reform, and have the greatest tendency to form soot rather than the desired product gas. Further, they are more likely to contain large amounts of sulfur. xe2x80x9cLogisticxe2x80x9d fuel is an extreme case. It is a low-grade, high sulfur diesel fuel that may be the only fuel available to the military in the field. While reciprocating and turbine ICEs operate directly on logistic fuel, fuel cell power plants require extensive fuel processing capability, resulting in additional size and weight.
The method of sulfur removal depends on both the reforming system and the type of fuel. If the reforming reaction uses a catalyst, then the sulfur is typically removed from the feedstock prior to reforming. Hydrodesulfurization is the classic means used for liquid hydrocarbons. Hydrogen separated from the product gas stream is reacted with the fuel over the catalyst to convert the sulfur compounds to hydrogen sulfide. The hydrogen sulfide is then removed by passing the stream through a zinc oxide bed. Activated charcoal filtration is sufficient to remove sulfur from natural gas before reforming. Non-catalytic POX reformers tolerate sulfur in the fuel, and convert it to hydrogen sulfide that can be removed from the product gas with a zinc oxide bed.
Since low temperature fuel cells can only utilize hydrogen and do not tolerate over 50 ppm CO, shift conversion and selective oxidation stages must be added to increase hydrogen and decrease CO levels. The situation is simpler for high temperature fuel cells. At 600xc2x0 C. to 1000xc2x0 C., CO and moderate quantities of hydrocarbons are reformed at the nickel anode surface using the steam, CO2 and heat from the power generation reaction. The reforming process only needs to break down the heavy hydrocarbons into a mix of gasses that the SOFC can utilize directly or reform internally without soot formation. High-temperature fuel cell systems can therefore use the product gas from steam, autothermal and POX reformers directly.
Startup characteristics are often important in fuel cell power plants operating on hydrocarbon and alcohol. A certain amount of time is needed to start a reformer to generate hydrogen, and high temperature fuel cells require time to heat to operating temperature regardless of the availability of fuel. This delay necessitates an interim power source such as a battery or ICE for applications that require immediate response, such as vehicle propulsion or emergency power.
ICEs include turbine, reciprocating piston or other machines that compress air, heat the air by reacting fuel with the oxygen in the air, and expand the heated air to produce work. The theoretical amount of fuel required to consume the oxygen in the air is termed the stoichiometric quantity. Typically, the amount of fuel added is less than the stoichiometric quantity (a lean mixture), since this makes the most efficient and economical use of the fuel. Fuel in excess of the available oxygen (a rich mixture) is discharged in the exhaust and produces no useful work. The composition of excess hydrocarbon fuel, however, is changed by the combustion process. Rich mixture exhaust contains hydrogen, carbon monoxide, and small amounts of hydrocarbons in addition to nitrogen and water vapor. Oxides of nitrogen (NOX), typical pollutants produced by lean mixtures, are suppressed by the reducing environment created by the rich mixture. In addition, sulfur compounds are converted to hydrogen sulfide. The overall result of rich ICE operation with hydrocarbon fuel is shaft work and almost complete conversion of the excess fuel into product gas containing hydrogen and CO. One of the specific problems with a rich running ICE is the production of soot. The theoretical rich soot formation limit for fuel with a stoichiometric ratio of 14.65 is 5.5, but in a real piston ICE soot formation occurs at higher ratios.
Use of an air/fuel mixture richer than stoichiometric in an ICE is a known technique to produce combustible gas. U.S. Pat. No. 4,041,910 by Houseman, assigned to NASA, describes a multicylinder engine in which the exhaust from two rich-running cylinders is used to fuel six lean-running cylinders. This avoids the oxide of nitrogen formation peak near stoichiometric operation, while providing complete fuel combustion. Houseman states that soot-free operation as low as 6.5 can achieved by adding water or steam, recycling the water-containing exhaust from the lean-running cylinders, or vaporizing and thoroughly mixing the fuel with heated air. U.S. Pat. No. 5,339,634 by Gale et. al., assigned to Southwest Research Institute, shows a similar system. In Gale et. al. a shift conversion catalyst is used to increase the hydrogen content of the rich-running cylinder exhaust. This exhaust is then mixed with additional fuel and air and fed to the lean-running cylinders where the hydrogen extends the lean limit. Neither of these patents contemplates using the rich exhaust as fuel for fuel cells.
U.S. Pat. No. 6,276,473 B1 by Zur Megede shows fuel cells and ICEs combined in an integrated vehicle power plant. It does not, however, utilize the ICE as a fuel processor for the fuel cell. Instead, it uses it as a means of providing immediate vehicle motion, as a heat source to warm the fuel cell to operating temperature, and as a supplemental power source after warm-up. The ICE and fuel cell both use a common hydrogen fuel source.
The present invention is a means for generating power from hydrocarbon and alcohol fuels in a power plant that integrates an ICE and a fuel cell. The ICE is operated with a rich hydrocarbon or alcohol fuel mixture to produce shaft power and an exhaust stream containing a mixture of gasses including hydrogen, carbon monoxide, and traces of hydrocarbons. The fuel cell then electrochemically oxidizes this product gas at the anode to produce electric power, while reducing oxygen at the cathode. The depleted fuel cell product gas and air exhaust streams may be handled in several ways. The prior art approach is to mix and combust the streams in an afterburner to produce process heat and eliminate exhaust pollutants. This invention includes additional productive uses for the depleted product gas stream. In one, a portion of the depleted product gas stream is recycled and combined with the ICE inlet fuel-air mixture to supply water vapor for soot suppression. In another, the depleted product gas stream is mixed with air to form a lean mixture that is burned in a separate ICE to produce shaft power and serve as an afterburner. The separate ICE may also be a section of the same machine used to process fuel. An example is to use one or more dedicated cylinders in a multi-cylinder reciprocating ICE that also includes fuel processing cylinders as an afterburner.
The present invention has a number of objectives. First, it employs the high peak temperatures in the ICE cycle to decompose the hydrocarbons and alcohols and hydrogenate sulfur compounds without catalysts. In particular, difficult feedstock such as xe2x80x9clogisticxe2x80x9d fuel may be processed. At the same time, oxide of nitrogen formation is strongly suppressed through the reducing effect of excess fuel. Reciprocating ICEs are particularly effective in achieving high peak combustion temperatures (on the order of 2000xc2x0 C.) while maintaining the engine components at relatively low temperatures compatible with ordinary materials. Second, thermodynamic advantages are gained. The gas expansion work produces shaft power and reduces the gas temperature so that the exhaust temperature is on the order of 700xc2x0 C. Like electric power, shaft power is thermodynamically the highest grade of energy, and contributes to the overall system efficiency. Third, system operation is enhanced. ICEs start in seconds and, while the system warms up, produces immediate shaft power that may be used for a number of purposes including vehicle propulsion and emergency electric power generation. The hot exhaust serves to heat the balance of the fuel processing system and start the electrochemical power generation process. The ICE may be controlled such that startup operation is near stoichiometric to maximize shaft power output and minimize fuel waste and exhaust pollution while the system is heated, and then shifted to rich operation. In general, the ICE facilitates system control. Rotational speed, throttle position and fuel-air ratio may be varied over a wide range to control the composition and flow rate of product gas. Fourth, the invention utilizes mature, low cost ICE technology that is supported by a ubiquitous manufacturing, service and fuel supply infrastructure. This facilitates earlier widespread fuel cell application with the attendant environmental and energy conservation benefits.
In summary, rich-running ICEs are more that simple replacements for conventional reformers in fuel cell systems. The integration of ICEs and fuel cells of the present invention is a novel and synergistic combination that forms a power plant with the energy efficiency and environmental advantages of fuel cells together with the fast response and fuel flexibility of ICEs.
Upon examination of the following detailed description the novel features of the present invention will become apparent to those of ordinary skill in the art or can be learned by practice of the present invention. It should be understood that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only. Various changes and modifications within the spirit and scope of the invention will become apparent to those of ordinary skill in the art upon examination of the following detailed description of the invention and claims that follow.